Comparative Study of the Microstructure and Tensile Properties of Ni-Al Alloys with Fe and Cr Additions
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EXPERIMENTAL Crystals of two compositions, Ni-32at.% Al-5at.% Fe and Ni-32at.% Al-12at.% Cr grown by directional solidification have been studied. The Ni-Al-Fe alloy was annealed at II 000 C for two hours prior to deformation. The Ni-Al-Cr alloy underwent hot isostatic pressing at 12600 C to reduce porosity. After that it was solution heat treated at 1260°C for 4 hrs and subsequently aged at 800'C for 24 hrs. Tensile tests were performed in the 20 - 7500 C temperature range at a strain rate of 3.3. 10 s- for the 0.2% proof stress and 1.6. 10-3 s-1 thereafter. Specimens were furnace cooled after deformation. TEM samples have been cut normal to the tensile axis using a spark erosion machine and jet electropolished. A Philips CM30 transmission electron microscope (TEM) operating at 300kV, JEOL 200CX TEM and JEOL 2000FX TEM operating at 200kV were used for the microstructural studies.
RESULTS AND DISCUSSION Initialmicrostructure
Ni-Al-Fe alloy A typical microstructure of the Ni-Al-Fe alloy can be seen in Fig. 1. Electron diffraction patterns and EDX analysis have shown that the alloy mainly consists of two phases, the P3-phase, which KK5.18.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
is Ni-rich and contains about 60 at.% of Ni, and the y'-phase. The alloying element, Fe, is present in both the P3 and 'y'phases with the concentration being slightly higher in the P-phase. A very small fraction of the martensitic phase with the Li 0 structure (not seen in Fig. 1) is present in the alloy [8]. Fragmentation of both the P3 and y' phases occurred during annealing (see Fig. 1). Sub-boundaries introduce some misorientation in the initially single-crystalline P and y' phases. Some of these boundaries are low angle dislocation boundaries and are formed by a dislocation climb process which can easily occur at the temperature where annealing has been performed (approx. 0.7 Tm). It is important to note the quite high dislocation density (about 1.109 cm-2) in both phases. During annealing and subsequent cooling the Li 0 phase undergoes a reverse martensitic transformation to the P3-phase and then the phase transformation 03--* -y'occurs. The volume changes during the above transformations cause stress fields and their relaxation leads to the dislocation substructure observed [8]. The fact that many dislocations are present within the grains after annealing is consistent with a low dislocation mobility. Ni-Al-Cr alloy
The Ni-Al-Cr alloy has a more complex microstructure than the Ni-Al-Fe alloy and is shown in Fig. 2. Three phases have been identified in the alloy namely, 13-phase, ,'f-phase and a-Cr phase (bcc). The solubility of Cr is known to be low in the 13phase [9], hence most of the Cr precipitates in the a-Cr phase and only about 2-5% of Cr is present as a solid solution in the 13and 'Y'phases. Two types of Cr precipitates are observed in the 13- matrix, large inclusions, about 0.5ýLtm in size, and very fine precipitates, about 80nm across (Fig. 2). The lattice parameter of the bcc Cr phase is al
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